Conventionally, there has been known an energy storage device which includes an electrode assembly formed of: a positive electrode having a positive composite layer and a positive composite layer non-forming portion; a negative electrode having a negative composite layer and a negative composite layer non-forming portion; and a separator interposed between the positive electrode and the negative electrode, wherein the positive composite layer non-forming portion and the negative composite layer non-forming portion are made to project in directions opposite to each other.
A material (for example, polypropylene, polyethylene or the like) used for forming the separator is shrunken to some extent due to heat. Accordingly, to prevent short circuit between the positive electrode and the negative electrode caused by thermal shrinking of the separator, separator is disposed such that the separator projects longer than edge portions of the positive composite layer and the negative composite layer.
Patent document 1 discloses a secondary battery having the following configuration (paragraphs 0010, 0027 and the like). By focusing on a phenomenon that when heat generated by an overcharge or the like is accumulated in the secondary battery, a temperature of a positive electrode non-coated portion side is liable to be higher than a temperature of a negative electrode non-coated portion side, a projecting amount of a separator on the positive electrode non-coated portion side is set twice or more as large as a projecting amount of the separator on the negative electrode non-coated portion side so that short circuit can be properly prevented even when the temperature is elevated.